Chirality Control and Its Memory at Microphase-Separated Interface of

Letter pubs.acs.org/macroletters

Chirality Control and Its Memory at Microphase-Separated Interface of Self-Assembled Chiral Block Copolymers for Nanostructured Chiral Materials Ming-Chia Li,† Naoki Ousaka,‡ Hsiao-Fang Wang,§ Eiji Yashima,*,‡ and Rong-Ming Ho*,§ §

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan † Department of Biological Science and Technology, National Chiao Tung University, Hsinchu 300, Taiwan ‡

S Supporting Information *

ABSTRACT: Here, we show the induced chirality of an achiral chromophoric dye as a joint of polylactide-containing chiral block copolymers (BCPs*) driven by self-assembly, giving the achiral dyes preferentially arranged in a one-handed helical array at the microphase-separated interface. This helical arrangement of the achiral dyes can be “memorized” after hydrolysis of the polylactides in the BCPs* and serves as a chiral template for further chirality induction of different achiral dyes, probably through attractive aromatic π−π interactions at the interface, producing nanostructured chiral materials with tunable circular dichroism signals at desired wavelengths.

H

In the present study, we envision that achiral chromophoric dyes, such as perylene, could be introduced as a joint of the BCPs* (Scheme 1) while maintaining its supramolecular columnar structure through which the achiral dyes could be arranged in a preferred, one-handed helical array at the microphase-separated interface of self-assembled BCPs*, giving an induced circular dichroism (ICD). The chiral PLLA blocks are degradable and can be removed by hydrolysis, but the achiral dyes are expected to retain their helical arrangement in a well-ordered nanoporous PS matrix, thus, showing optical activity due to a memory effect in the chiral space.2c,i,9 Consequently, the helically arranged chromophoric dyes with a chiral memory can serve as chiral hosts to further helically assemble with different achiral dyes as guests via π−π interactions, thus, providing a unique nanostructured chiral ensemble, giving a tunable left- and right-handed circular polarization (LCP and RCP) along with optical transparency in the desired wavelength region. The self-assembly and subsequent chiral memory in this study rely mainly on the initial microphase separation of chiral BCPs followed by the vitrification of the PS matrix, which is different from the π−π stacking and hydrogenbond-assisted supramolecular chiral memory systems.9 Chirality induction at microphase-separated interface of BCP*: To acquire a well-defined microphase-separated phase, a coassembling approach was carried out by blending helix-forming

omochirality is significantly important in living systems, leading to elaborated biological functions that rely on molecular, macromolecular and hierarchical chiralities.1 Chirality control of polymer and supramolecular assemblies from small to large-length scale is an attractive goal due to its appealing applications in materials science.2,3 We recently reported that chirality transfer at different length scales can be achieved by self-assembling of polylactide-containing chiral homopolymers and chiral block copolymers (BCPs*).4 Selfassembly is a thermodynamically spontaneous organization of molecules or supramolecules into stable, well-defined aggregates by secondary interactions (i.e., noncovalent interactions).5 Among the self-assembled architectures, helical morphology is probably the most fascinating texture in nature; for instance, the right-handed α-helix for proteins and the right-handed double-helix for the A- and B-form DNAs are established by the homochirality of constituted components, L-amino acids and nucleobase-bound D-sugars, respectively, via specific hydrogen bonding. By introducing chirality into synthetic molecules, helical textures of different length scales can be obtained by self-assembly through the interplay of secondary interactions.6 Macromolecules composed of chiral entities through covalent or noncovalent bonding interactions have been extensively utilized for self-assembly to construct one-handed helical morphologies.7 During the course of our studies on the self-assembly of BCPs*, we recently found a unique helical nanostructured phase (denoted as H* phase) composed of hexagonally packed poly(L-lactide) (PLLA) helices in a polystyrene (PS) matrix.8 © XXXX American Chemical Society

Received: July 11, 2017 Accepted: August 16, 2017

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DOI: 10.1021/acsmacrolett.7b00493 ACS Macro Lett. 2017, 6, 980−986

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Scheme 1. Chirality Control and Its Memory Followed by Subsequent Chirality Induction at Microphase-Separated Interface of Self-Assembled BCP*

Figure 1. (a) TEM micrograph of PS-perylene-PLLA-H*/PS-PLLA-H* blends. (b) One-dimensional SAXS profiles of PS-perylene-PLLA-H*/ PS-PLLA-H* blends (blue line) and PS-perylene-PDLA-H*/PS-PDLA-H* blends (red line). (c) CD (solid line), LD (dash line), and the corresponding absorption spectra of PS-perylene-PLLA-H*/PS-PLLA-H* (blue line) and PS-perylene-PDLA-H*/PS-PDLA-H* blend thin films (red line). (d) PLM observations of PS-perylene-PLLA-H*/PS-PLLA-H* blends, measured at the 0° (upper) and 90° position (lower). The BCP* blends are mixed at a ratio of 30/70 (w/w).

polystyrene-b-poly(L-lactide) (PLLA) (PS-PLLA-H*) ( f PLLAv = 0.35) with PS-perylene-PLLA-H* ( f PLLAv = 0.35; see Supporting Information and Figures S1−3 for details). As shown in Figure 1a, well-developed helical nanostructures can be obtained

in the PS-perylene-PLLA-H*/PS-PLLA-H* blends via the coassembly. The H* phase of the blends was further evidenced by the characteristic SAXS reflections at the relative q values of 1:√4:√7 (Figure 1b), suggesting hexagonally packed 981


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( f PDLAv = 0.25)/PS-PDLA-HC (f PDLAv = 0.26) composed of a different PS/PLA block length (Table S1), resulting in the formation of a well-defined symmetric hexagonal cylinder (HC) phase as supported by the TEM images (Figures 2a and S5, respectively) and the reflections at the relative q values of 1:√3:√7 in the SAXS profile (Figure 2b) of the blends. Similar results are found in the H* phase (Figure 1c), where the HC-forming blends exhibit split-type ICDs with perfect mirror images to each other, to which the LD (Figure 2c) and LB (Figure 2d) contributions could be negligible. These results clearly indicate that, not only the asymmetric H* phase, but also the symmetric HC phase can induce a preferred, one-handed helical array of the achiral perylenes at the microphase-separated interface of the BCPs* during the coassembling processes. As shown in Figures S6 and S7, significant differences between the CD and absorption spectra of the long-axis molecular plane of the perylene moieties for the H* and HC phases resulting from the blending of a series of the PS-perylene-PLLA and PS-PLLA at different mixing ratios can be found. The absorption band intensity ratios at 525 and 487 nm (I525/I487) for both the H* and HC phases decrease as the concentration of PS-perylene-PLLA increases. Also, a significant bathochromic shift can be observed for the H* blends; that is, the characteristic behavior for the formation of J-aggregated perylenes.12 Accordingly, the perylene residues form the J-aggregates in the H* phase through a shifting and twisting mechanism,4a whereas the perylenes in the HC phase form a

helices. Similar TEM and SAXS results were also obtained for the enantiomeric blends, PS-perylene-PDLA-H*/PS-PDLA-H* (Figures S4 and 1b, respectively). The CD spectrum of the PS-perylene-PLLA-H*/PS-PLLA-H* blends exhibits split-type ICDs with the first negative and second positive Cotton effects at 548 and 487 nm, respectively, in the achiral perylene absorption regions, and the CD spectrum of PS-perylene-PDLAH*/PS-PDLA-H* blends shows the perfect mirror image Cotton effects and identical absorption spectra as anticipated (Figure 1c). Based on the exciton-coupled CD method,10 the perylene moieties of the PLLA- and PDLA-containing blends with the H* phases can be assigned to have left- and righthanded helical arrays at the microphase-separated interface, respectively. It has been reported that the observed CD of solid samples may include components arising from linear dichroism (LD) and linear birefringence (LB) due to their macroscopic anisotropy.11 We noted that the LD contribution to the CD spectra of the films could be negligible because the optical densities from the LD is less than ±0.005 (Figure 1c). In addition, the fact that the corresponding polarization light microscopy (PLM) images of the BCP* blends show a red-violet color, which remains unchanged after rotating the sample by 90° (Figure 1d), reveals negligible contribution of the LB toward the CD spectra based on the sensitive tint of Newton’s color sequence. More interestingly, a similar one-handed helical array of the achiral perylenes at the microphase-separated interface can be found in the PS-perylene-PLLA-HC (f PLLAv = 0.25)/PS-PLLAHC (f PLLAv = 0.25) blends and the PS-perylene-PDLA-HC

Figure 2. (a) TEM micrograph of PS-perylene-PLLA-HC/PS-PLLA-HC blends. (b) One-dimensional SAXS profiles of PS-perylene-PLLA-HC/ PS-PLLA-HC blends (blue line) and PS-perylene-PDLA-HC/PS-PDLA-HC blends (red line). (c) CD (solid line), LD (dash line), and corresponding absorption spectra of PS-perylene-PLLA-HC/PS-PLLA-HC (blue line) and PS-perylene-PDLA-HC/PS-PDLA-HC blend thin films (red line). (d) PLM observations of PS-perylene-PLLA-HC/PS-PLLA-HC blends, measured at the 0° (upper) and 90° position (lower). The BCP* blends are mixed at a ratio of 30/70 (w/w). 982


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the chiral polylactide segments results in the formation of voids or pores in the HC-forming films, which may cause artificial effects during the CD measurements. In fact, the hydrolyzed films show PLM images with different colors after rotation by 90° (Figures S12 and S13), suggesting that the LB contributions might be unavoidable. To overcome this LB effect arising from the periodic variation in the refractive index between the hydrolyzed BCP microphase-separated domains (i.e., PS) and air,15 the hydrolyzed thin films were treated with a styrene oligomer to fill the pores with styrene oligomers for index-matching, through which the corresponding PLM images show yellow (upper, Figure 3c) to pink color (lower, Figure 3c) and remain unchanged regardless of the sample rotation, indicating that the LB effect can be significantly diminished. Quite interestingly, the hydrolyzed thin films show apparent mirror-image Cotton effects in the perylene chromophore regions, even in the absence of the chiral entity (Figure 3d), leading to a unique chirality memory effect2i,9 that occurs at the microphase-separated interface, indicating that the dangling perylenes located at the inner wall of the nanoporous PS maintain their optical activities due to the vitrification of the PS rather than the π−π interaction of joints, which would result in reduced structural ordering.16 Therefore, the helically assembled perylene aggregates completely lost their chiral memory after annealing the thin-film at a higher temperature (180 °C) above the Tg of the PS (∼100 °C; dark line, Figure 3d). The disappearance of the chiral memory is accompanied by a significant change in the absorption spectrum resulting from the dissociation of the stacked perylene moieties17 due to the

face-to-face stacking of H-aggregated perylenes through a twisting only mechanism.12g Similar absorption spectral variations can also be found for the PS-perylene-PDLA/ PS-PDLA blends with the H* and HC phases (Figures S8 and S9, respectively). Memory of helical assemblies at the interface in the microphase-separated BCP*: As reported previously, the H* phase is a long-lived metastable phase, whereas the HC phase is intrinsically a stable phase.8 Consistent with this hypothesis, the HC-forming BCP* was used as a representative one for demonstrating the memory of the achiral perylenes at the microphase-separated interface after removal of the chiral polylactide segments. The chiral polylactide segments in the self-assembled HC phase can be totally removed by alkaline hydrolysis,13 as evidenced by complete disappearance of the signals of the chiral polylactide segments in the IR and NMR spectra (Figures S10 and S11, respectively). Figure 3a shows the TEM image of the HC-forming PS-perylene-PLLA/ PS-b-PLLA-HC blends after hydrolysis followed by a templated sol−gel reaction at which the mass−thickness contrast is reversed compared to that in Figure 2a due to the high-mass contrast of silicon dioxide, revealing successful templating of the cylindrical texture.14 The 1D SAXS profile of the hydrolyzed film (Figure 3b) reveals the cylinder-forming nanochannels that are hexagonally packed. These results suggest that the self-assembled cylindrical morphology can be preserved, even after hydrolysis of the chiral polylactide segments. Similar SAXS result was also obtained for the HC-forming PS-perylene-PDLA/PS-b-PDLA blends after hydrolysis of the PDLA segments (Figure 3b). Removal of

Figure 3. (a) TEM micrograph of PS-perylene-PLLA-HC/PS-PLLA-HC blends after hydrolysis followed by templated sol−gel reaction. (b) Onedimensional SAXS profiles of PS-perylene-PLLA-HC/PS-PLLA-HC (blue line) and PS-perylene-PDLA-HC/PS-PDLA-HC blends (red line) after hydrolysis. (c) PLM observations of PS-perylene-PLLA-HC/PS-PLLA-HC blends after hydrolysis (upper), followed by pore-filling with styrene oligomer (lower). (d) CD (solid line), LD (dash line), and the corresponding absorption spectra of PS-perylene-PLLA-HC/PS-PLLA-HC (blue line) and PS-perylene-PDLA-HC/PS-PDLA-HC blends (red line) after hydrolysis, followed by pore-filling with styrene oligomers. The results indicated by the black line are the ones from above after further post-annealing at 180 °C for 1 min. The BCP* blends are mixed at a ratio of 30/70 (w/w). 983


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Figure 4. (a) SEM micrograph of PS-perylene-PDLA-HC/PS-PDLA-HC blends after hydrolysis by association with IR-1061. (b) PLM observations of PS-perylene-PDLA-HC/PS-PDLA-HC blends after hydrolysis followed by association with IR-1061 and then pore-filling with styrene oligomer, measured at the 0° (upper) and 90° position (lower). (c) CD (solid line), LD (dash-dot line), and the corresponding absorption spectra of PS-perylene-PDLA-HC/PS-PDLA-HC blends after hydrolysis (red line) followed by association with IR-895 (purple line) and IR-1061 (green line) and then pore-filling with styrene oligomers, respectively.

due to the absorption regions of IR-895 while the negative Cotton effect arising from the template perylenes remains. Also, the LD contributions to the observed CD spectra could be negligibly small based on their weak LD signals (Figure 4c). In summary, we have developed versatile chiral nanostructured materials showing tunable CD signals at the desired wavelengths using self-assembled BCPs* composed of PS and optically active degradable polylactides joined by achiral dyes that retain its unique chiral phase along with helically arranged chromophoric dyes at the microphase-separated interface. Combining the knowledge from this aspect, the transcription of hierarchical chirality through the interaction of helically assembled chromophoric dyes at solid surfaces offers one of the promising ways to produce not only synthetic threedimensional chiral templates with tunable optical activities from visible to near-infrared regions, but also chirality-imprinted nanostrucutures with enantioselective catalytic and separation abilities.2a−c

morphological deformation of the PS matrix that produces a disorder texture. Helical assembly of achiral dyes at the interface with a chiral memory: The helically arranged perylene aggregates induced and memorized at the microphase-separated interface are anticipated to provide a versatile chiral template for further chirality induction of different achiral dyes through aromatic interactions at the interface. To this end, a near-IR chromophoric dye, 4-[2-[2-chloro-3-[(2,6-diphenyl-4H-thiopyran-4ylidene)ethylidene]-1-cyclohexen-1-yl]-ethen-yl]-2,6-diphenylthiopyrylium tetrafluoroborate (IR-1061) with absorption bands different from those of perylene was selected as an achiral guest. It is noted that a continuous cylindrical nanochannel is required to accommodate the guest molecules through pore-filling driven by a capillary force. As shown in the SEM image with three-dimensional view (Figure 4a), connected cylindrical pores can be clearly identified in the composite thin film with IR-1061, which shows rotation-independent PLM images (Figure 4b), indicating the negligible LB effect on the CD measurements. The CD spectrum of the composite thin film derived from the PS-perylene-PDLA-HC/PS-b-PDLA-HC blends after hydrolysis followed by association with IR-1061 exhibits an intense negative Cotton effect in the IR-1061 chromophore regions (900−1150 nm), which are quite different from the helically arranged perylene bisimides with a chiral memory in its absorption and CD regions (Figure 4c). The absorption bands due to the perylene moieties become broadened upon the association with IR-1061, suggesting the attractive aromatic interactions between the guest and host molecules,18 resulting in the formation of a further one-handed helical array of IR-1061 along the helically arranged perylene template. Another near-IR chromophoric dye, dimethyl{4-[1,7,7-tris (4-dimethylaminophenyl)-2,4,6-heptatrienylidene]-2,5-cyclohexadien-1-ylidene} ammonium perchlorate (IR-895) with a different absorption region was also found to associate with the helically arranged perylene residues, thus, giving an ICD around 600−1050 nm

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00493.

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Experimental details (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ming-Chia Li: 0000-0002-0361-3334 Eiji Yashima: 0000-0001-6307-198X Rong-Ming Ho: 0000-0002-2429-7617 Notes

The authors declare no competing financial interest. 984


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ACKNOWLEDGMENTS R.-M.H. would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 105-2119M-007-011. This work was also supported in part by Grant-inAid for Scientific Research (S) from the Japan Society for the Promotion of Science No. 25220804 (E.Y.) and by the Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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